Generally, this invention is directed toward a method for preparing high quality lightweight paper having increased opacity. The method employs a structured latex having both reinforcing and opacifying characteristics.
In the manufacture of papers, especially lightweight paper, a major concern is the opacity of the paper. Generally, opacity is provided by conventional fillers such as clay, calcium carbonate and titanium dioxide. Unfortunately, these fillers can adversely affect the mechanical properties of the paper if too much filler is attempted to be incorporated to obtain the desired opacity.
Attempts have been made to offset the reduction in mechanical properties through the use of various formulations of latexes having increased strength. However, improved methods of obtaining good opacity or light scattering characteristics without reducing mechanical properties are continually being sought. The problem is difficult to solve because opacity and strength are generally inversely proportional characteristics. The problem is particularly acute in lightweight papers where the amount of material that can be employed to form the paper is minimal.
It has been discovered that by specifically engineering a structured latex particle having a core/shell morphology that both characteristics of opacity and mechanical strength can be satisfied. This is accomplished by preparing the latex particle such that it not only reinforces or binds the constituents of the paper but also scatters light to provide its own measure of opacity to the paper.